The popup is a great upselling tool that will 'pop-up' after the user adds an item to their basket.
Firstly, to set the global popup: In the sidebar, go to Competitions > Popup Settings.
Tick the box "Redirect no button to basket" to make the 'Not Today' button take customers straight through to the basket instead of remaining on the competition page.
Use the drop-down to select a product/competition which the 'Yes' button will link to.
Then, create the popup by adding an image [Add Media] and text to the editor box and Save Changes.
Tick Use Global Popup to use the pop up you created above ^.
Or create a different popup here that will appear only on this competition.